This invention relates to planar heating element comprising thermoplastic resin as a supporter and electrically conductive powder unevenly distributed in the neighborhood of (and on) the surface thereof. Heretobefore, planar heating elements have been used in various fields in order to heat or maintain constant temperatures over wide areas or for example, as heaters for melting snow, preventing freezing of roads or water conduits, maintaining or elevating the temperature of chemical apparatus, or cultivating.
Known planar heating elements to be applied in above various fields are as follows.
A. a heating element produced by laminating corrosion-proof aluminum foil or copper foil of 2 - 100 .mu. in the thickness onto an insulating heat proof plastic film, then etching the metal foil.
B. a heating element produced by adhering carbon powder onto a plate made of glass fiber, then laminating an elastomeric or plastic insulating cover.
C. a heating element produced by weaving carbon fiber.
D. a heating element produced by molding elastomer or plastic resin and carbon powder.
Although (A) can give an extremely thin heater having high power in spite of small area, it as many shortcomings such as weakness to bending and the difficulty in large capacity heating. (B) has the such shortcomings as low flexibility, great changes in resistance as a result of bending, difficulty in maintaining constant resistance and weakness to pressure. (C) has the same shortcomings as (B) and carbon fiber is quite expensive. (D) is a good processibility but has a critical shortcoming. In (D), a great amount of carbon powder is necessary in order to obtain a resistance value suitable for operation under low voltage such as 100 V or less. In some case, 50 - 60 % by weight of carbon powder must be added. However, in a case of adding such a great amount of carbon powder to thermoplastic resin, it is quite difficult to mold them and the obtained heater has less flexibility, accordingly the use is quite restricted.
Planar heating elements of the above mentioned (A) - (D) types have many shortcomings and can be applied only for use of small size and small capacity. In addition, known planar heating element have a characteristic that when using them at an elevated temperature or long time, their resistance values drop in accordance with the negative resistance of carbon itself. Under an elevated temperature, higher electric current is necessary, so, finally the heaters are destroyed by over heat. Accordingly, the known planar heating elements require various means for temperature detection and controlling.